The invention relates to a system for the reduction of laser equivalent surface area and for optical protection especially against aggression by laser beams.
In an aimed optical system, the detector (the retina of the eye or an electronic image sensor, for example of the CCD type), located in the vicinity of a focal plane of the system is liable to reflect incident light in the direction of incidence. This phenomenon, which is commonly called the "cat's eye effect" is especially troublesome when the optical system has to be discreet. Indeed, the system can be easily detected and located at a distance by means of a simple divergent source associated with an imaging system. Once localized, the system may be attacked by means of a high power source which may result in the destruction of the user's retina or of the sensor.
The parameter used by those skilled in the art, which expresses the capacity of an optical system to reflect light along the direction of incidence, is called the laser equivalent surface area. The laser equivalent surface area of the naked eye is about 0.1 m.sup.2 while that of the eye associated with a standard pair of binoculars is some hundreds of m.sup.2. That of a CCD camera commonly attains several thousand of m.sup.2. These values may vary considerably depending on the optical quality of the system.
If it is desired to aim at an object while remaining discreet, which is especially the case in military applications, it is particularly important to be able to reduce the laser equivalent surface area of the system while lowering the quality and luminosity of the image given by the system to the least possible extent.
Furthermore, if the system has reduced laser equivalent surface area, the risk of localization will be lower. However it can never be strictly zero. It is therefore desirable that the system should also enable protection against laser aggression.
Two methods are used to reduce the laser equivalent surface area of an optical system.
The first method consists of the use of optical filters centered on the usual wavelengths of the lasers. The laser equivalent surface area of the system is therefore reduced for these wavelengths only. There is therefore a risk that this system may be detected by a source with an unusual wavelength or by an agile source.
The second method is applicable only to systems that comprise an electronic image sensor. It consists of the deposition, on the sensor, of a layer limiting the reflection of light on the surface of the sensor. This method has limited efficiency and cannot be applied to direct viewing systems (binoculars, etc.).